A photoacoustic imaging method, which is a bioinformation acquisition method, is a method of detecting an acoustical wave induced in the internal portion of a living body by radiating a pulsed laser light to the living body, thereby imaging the three-dimensional structure of the internal portion of the living body. The acoustical wave is generated by the radiation of the pulsed laser light to a test object in a living body to cause the thermal expansion of the test object in the internal portion of the living body. Moreover, by changing the wavelength of the wavelength of the pulsed laser light, it is possible to visualize the distributions of specific substances, such as hemoglobin and glucose in blood, having an absorption band of the wavelength. Consequently, because a potential tumor, such as the abnormal growth of new blood vessels, can be non-invasively determined, the photoacoustic imaging method has been seen as a potential device for screening for breast cancer or the early detection thereof in recent years.
A conventional concrete procedure of the photoacoustic imaging method is disclosed in, for example, Published Japanese Translation of a PCT Application No. 2001-507952 as follows.
(1) Two-dimensionally arranged electromechanical conversion elements (transducers) are located on the surface of a test body, and single pulse electromagnetic energy is radiated to the test body.
(2) Just after the radiation of the electromagnetic energy, the signal received by each electromechanical conversion element is sampled to be stored.
(3) As to a point r′ in the test body to be visualized, the delay time necessary for an acoustical wave to reach the position r of each electromechanical conversion element i from the point r′ is calculated, and the signal of each electromechanical conversion element i corresponding to the calculated delay time is added to one another to be set as the image value at the point r′.
(4) The step (3) is repeated to each point r′ to be imaged.
Moreover, Japanese Patent Application Laid-Open No. 2005-021380 discloses the method of reconstructing both of a photoacoustic image and an ordinary ultrasound echo image by using common one-dimensionally arranged electromechanical conversion elements, and the configuration of arranging an illumination system using glass fibers between the one-dimensionally arranged electromechanical conversion elements. Since the method disclosed in this Japanese Patent Application Laid-Open No. 2005-021380 uses the one-dimensionally arranged electromechanical conversion elements, the method is required to repeat the reconstruction by mechanically moving the one-dimensionally arranged electromechanical conversion elements into the direction perpendicular to the arrangement direction of the transducers in order to reconstruct a three-dimensional image.
In order to reconstruct the three-dimensional image by using the photoacoustic imaging method, it is desirable to use two-dimensionally arranged electromechanical conversion elements in order to reduce the direction dependency of an image resolution. As the methods for obtaining a photoacoustic image in a wide area on the premise of the use of the two-dimensionally arranged electromechanical conversion elements, the following methods can be considered: (1) the method of arranging electromechanical conversion elements on the whole wide area, and (2) the method of locating a comparatively small-scale electromechanical conversion element group (a group composed of arranged electromechanical conversion elements) in a step and repeat system to perform mechanical scanning. However, the method (1) has the problem of the difficulty of commercial viability in cost owing to the scale enlargement of the receiving system of the method. Moreover, the method (2) has the problem of the occurrence of the unevenness of sensitivity between the central part and the end parts of the two-dimensionally arranged electromechanical conversion elements. Moreover, the method (2) has the problem of the waste of time to locate the electromechanical conversion element group to the next positions one by one in the step and repeat system.